Studies of the morphological effects of antimalarial drugs on malarial parasites yield important information on the site of action of the drugs. Changes in the structure of the malarial parasites furnish clues to the mode of drug action. In the current year we continued to investigate the mode of action of clindamycin and its N-demethyl-4'-pentyl analogue against P. knowlesi in rhesus monkeys. Clindamycin and its analogue appear to act on ribosomes of malarial parasites. We also investigated effects of immune serum on merozoites of P. knowlesi by electron microscopy. Immune serum agglutinated merozoites and these merozoites were unable to invade new erythrocytes. Merozoites agglutination was caused by the binding of surface coats on adjacent parasites. BIBLIOGRAPHIC REFERENCES: Aikawa, M., Miller, L.H. and Rabbege, J., Caveola-vesicle complexes in the plasmalemma of erythrocytes infected by Plasmodium vivax and Plasmodium cynomolgi: Unique structures related to Schuffner's dots. Am. J. Path. (In Press) 1975. Gutierrez, Y., Aikawa, M., Fremont, H.N. and Sterling, C.R., Experimental infection of Aotus monkeys with Plasmodium falciparum: Light and electron miscroscopic changes. Ann. Trop. Med. Parasitol. (In Press) 1975.